1. Field of the Invention
The present invention concerns the field of transmission on optical fiber.
2. Description of the Prior Art
The index profile of an optical fiber is generally described by the shape of the graph of the refractive index as a function of the radius of the fiber. The distance r from the center of the fiber is conventionally plotted on the abscissa axis and the difference between the refractive index of the fiber and the refractive index of the cladding is conventionally plotted on the ordinate axis. The expressions "step", "trapezium" and "triangle" index profiles are therefore used for graphs which are respectively step-, trapezium- and triangle-shaped. These curves are generally representative of the theoretical or set point profile of the fiber, and fiber manufacturing constraints can lead to a substantially different profile.
In new high bit rate wavelength division multiplex transmission networks it is advantageous to control chromatic dispersion, especially for per channel bit rates of 10 Gbit/s and above. The objective is to restrict widening of the pulses by obtaining substantially no cumulative chromatic dispersion for all wavelength values of the multiplex. A cumulative dispersion value of a few hundred ps/nm is generally acceptable. It is also beneficial to avoid zero values of chromatic dispersion in the vicinity of the wavelengths used in the system, because non-linear effects are greater at these values. Finally, it is also beneficial to limit the chromatic dispersion slope over the range of the multiplex to restrict or restrict distortion between the channels of the multiplex.
Stepped index fibers are conventionally used as line fibers in fiber optic transmission systems. The assignees of the inventors sell an ASMF 200 stepped index monomode fiber having a chromatic dispersion cancellation wavelength .lambda..sub.0 in the range from 1 300 nm to 1 320 nm and a chromatic dispersion less than or equal to 3.5 ps/nm/km in a range from 1 285 nm to 1 330 nm and of 17 ps/nm.km at 1 550 nm. The chromatic dispersion slope at 1 550 nm is in the order of 0.06 ps/nm.sup.2.km.
Dispersion shifted fibers (DSF) have also appeared on the market. These fibers have substantially no chromatic dispersion at the transmission wavelength at which they are used, which is generally different from the wavelength of 1.3 .mu.m for which silica shows substantially no dispersion. In other words, the non-zero chromatic dispersion of the silica is compensated--whence the use of the word "shifted"--by increasing the index difference .DELTA.n between the core of the fiber and the optical cladding. The index shift means that the wavelength for which there is substantially no chromatic dispersion can be shifted. It is achieved by introducing dopants into the preform, during its manufacture, for example by a conventional MCVD process which is not described in more detail here.
Non-zero dispersion shifted fibers (NZ-DSF+) have a non-zero and positive chromatic dispersion at the wavelength at which they are used. At these wavelengths these fibers have a low chromatic dispersion, typically less than 10 ps/(nm.km) at 1 550 nm, and a chromatic dispersion slope in the range from 0.04 ps/(nm.sup.2.km) to 0.1 ps/(nm.sup.2.km).
Using short lengths of dispersion compensating fiber (DCF) to compensate the chromatic dispersion and the chromatic dispersion slope in SMF or NZ-DSF+ fibers used as line fibers is conventional. One example of a transmission system of this kind in which the chromatic dispersion in an SMF type line fiber is compensated by DCF type fiber is described in "Dispersion compensating fibers and their applications" by M. Nishimura et al., OFC'96 Technical digest ThA1.
Such use of dispersion compensating fiber is also mentioned in "Large volume manufacturing of dispersion compensating fibers" by L. Gruner-Nielsen et al., OFC'98 Technical Digest TuD5. A disadvantage of this type of fiber is its high cost.
DCF type fibers are described in various patents. They have around the wavelength of 1 550 nm a negative chromatic dispersion to compensate the cumulative chromatic dispersion in the line fiber and can have a negative chromatic dispersion slope to compensate the positive slope of the chromatic dispersion of the line fiber. Thus U.S. Pat. No. 5,581,647 proposes (see table VI, examples B1 and B2), fibers having respective chromatic dispersions of -85 ps/(nm.km) and -100 ps/(nm.km) and chromatic dispersion slopes of -0.2 ps/(nm.sup.2.km) and -0.26 ps/(nm.sup.2.km). The ratio of the chromatic dispersion to the chromatic dispersion slope is then in the range from 425 nm to 500 nm. Such values are suitable for compensating chromatic dispersion and chromatic dispersion slope of SMF type line fibers.
U.S. Pat. No. 5,361,319 also proposes DCF type fibers. One index profile it proposes is a rectangular index profile with a trench and a ring; the fibers proposed are adapted to compensate chromatic dispersion and chromatic dispersion slope in SMF type fibers having a chromatic dispersion in the order of 15 ps/(nm.km) and a chromatic dispersion slope in the order of 0.06 ps/(nm.sup.2.km). One of the target values proposed for DCF type fiber is therefore -60 ps/(nm.km) for the chromatic dispersion and -0.24 ps/(nm.sup.2.km) for the chromatic dispersion slope.
U.S. Pat. No. 5,555,340 also proposes a DCF type fiber with an alpha profile and buried cladding. One of the points on the chromatic dispersion and chromatic dispersion slope graphs from FIGS. 5A and 5B of the above document corresponds to a fiber in which the ratio between the radii of the buried cladding and the alpha part is 0.4, with a diameter of 3.3 .mu.m for the alpha part. For this fiber the chromatic dispersion is in the order of -90 ps/(nm.km) and the chromatic dispersion slope is in the order of -0.6 ps/(nm.sup.2.km). The document does not specify the effective area of the fiber. Calculation of the properties of the fiber shows that the effective area is in the order of 10 .mu.m.sup.2.
U.S. Pat. No. 5,568,583 proposes a DCF type fiber with a rectangular profile and buried cladding; it specifies that the DCF type fiber is assumed to be used to compensate SMF type fiber having a dispersion in the order of 17 ps/(nm.km). The dispersion and dispersion slope values for various core diameters set out in tables 1, 2 and 3 of the above document do not agree with the proposed profile and cannot be clearly obtained for that profile.
EP-A-0 866 574 proposes a DCF type fiber having a rectangular profile with a ring. This fiber supports propagation not only in LP01 mode but also in the higher LP02 mode. Chromatic dispersion is very high, typically less than -200 ps/(nm.km) for LP02 mode.
U.S. Pat. No. 5,838,867 proposes DCF fibers having alpha profiles with buried claddings and with or without rings. The proposed fibers are suitable for compensating chromatic dispersion and chromatic dispersion slope of shifted dispersion fibers having zero chromatic dispersion in the range from 1 450 nm to 1 550 nm or from 1 450 nm to 1 650 nm and a positive dispersion slope. The target values proposed for the DCF type fibers are greater than -40 ps/(nm.km) for the chromatic dispersion, for example (FIG. 10) a chromatic dispersion of -35 ps/(nm.km) with a chromatic dispersion slope of -0.15 ps/(nm.sup.2.km) and (FIG. 11) a chromatic dispersion of -30 ps/(nm.km) with a chromatic dispersion slope of -0.39 ps/(nm.sup.2.km).
Finally, PCT/JP98/04066 proposes DCF type fibers having rectangular profiles and buried claddings. The proposed fibers are suitable for compensating chromatic dispersion and chromatic dispersion slope of the TRUE WAVE fibers sold by A.T.&T. Corporation, which have chromatic dispersions of 1.5 ps/(nm.km) to 4 ps/(nm.km). Curvature losses are not specified in the document. Calculating the properties of the fibers described in examples 1 and 2 for the ranges of index differences and of core part diameters given shows that the curvature losses are greater than 0.05 dB.
French patent application 99 02028 filed Feb. 18, 1999 with the title "Fibre de ligne pour systemes de transmission a fibre optique a multiplexage en longueurs d'onde" ("Line fiber for transmission systems using wavelength division multiplexed optical fiber") proposes a line fiber more particularly suited to dense wavelength division multiplex transmission, with a channel spacing of 100 GHz or less for a per-channel bit rate of 10 Gbit/s. For a wavelength of 1550 nm, this fiber has an effective area greater than or equal to 60 .mu.m.sup.2, a chromatic dispersion in the range from 6 ps/(nm.km) to 10 ps/(nm.km) and a chromatic dispersion slope less than 0.07 ps/(nm.sup.2.km).
The invention proposes a fiber suitable for compensating chromatic dispersion in the above type of fiber. It provides a fiber having strongly negative chromatic dispersion and chromatic dispersion slope so that compensation can be obtained with the shortest possible length of fiber. The fiber has low curvature losses and can easily be used as the line fiber in a transmission system.